R.M. De La Rue

A compact two-dimensional grating coupler used as a polarization splitter

We demonstrate a novel polarization splitter based on a two-dimensional grating etched in a silicon-on-insulator waveguide. The device couples orthogonal modes from a single-mode optical fiber into identical modes of two planar ridge waveguides. The extinction ratio is better than 18 dB in the wavelength range of 1530-1560 nm and the coupling efficiency is approximately 20%. The device is very compact and couples light only to transverse-electric modes of the planar waveguides. Therefore, it may be used in a polarization diversity configuration to implement a polarization insensitive photonic integrated circuit based on photonic crystal waveguides.

Optical and confinement properties of two-dimensional photonic crystals

We describe experiments on a quasi-two dimensional (2-D) optical system consisting of a triangular array of air cylinders etched through a laser-like Ga(Al)As waveguiding heterostructure. Such a configuration is shown to yield results very well approximated by the infinite 2-D photonic crystal (PC). We first present a set of measurements of the optical properties (transmission, reflection, and diffraction) of slabs of these photonic crystals, including the case of in-plane Fabry-Perot cavities formed between two such crystals. The measurement method makes use of the guided photoluminescence of embedded quantum wells or InAs quantum dots to generate an internal probe beam. Out-of-plant, scattering losses are evaluated by various means. In a second part, in-plane micrometer-sized photonic boxes bounded by circular trenches or by two-dimensional photonic crystal are probed by exciting spontaneous emission inside them. The high quality factors observed in such photon boxes demonstrate the excellent photon confinement attainable in these systems and allow to access the detail of the modal structure. Last, some perspectives for applications are offered.

Low-loss channel waveguides with two-dimensional photonic crystal boundaries

We have used transmission measurements to estimate the propagation loss of submicron channels defined in two-dimensional photonic crystals patterned into a Ga(Al)As waveguide. The measured propagation loss of the fundamental mode is indistinguishable from the material absorption, setting an upper limit of 50 cm−1 (2 dB per 100 μm). We also find that, provided the etching is deep enough, propagation losses of these photonic crystal waveguides are lower than those of ridge waveguides etched in the same run.

Tunable Delay Lines in Silicon Photonics: Coupled Resonators and Photonic Crystals, a Comparison

In this paper, we report a direct comparison between coupled resonator optical waveguides (CROWs) and photonic crystal waveguides (PhCWs), which have both been exploited as tunable delay lines. The two structures were fabricated on the same silicon-on-insulator (SOI) technological platform, with the same fabrication facilities and evaluated under the same signal bit-rate conditions. We compare the frequency- and time-domain response of the two structures; the physical mechanism underlying the tuning of the delay; the main limits induced by loss, dispersion, and structural disorder; and the impact of CROW and PhCW tunable delay lines on the transmission of data stream intensity and phase modulated up to 100 Gb/s. The main result of this study is that, in the considered domain of applications, CROWs and PhCWs behave much more similarly than one would expect. At data rates around 100 Gb/s, CROWs and PhCWs can be placed in competition. Lower data rates, where longer absolute delays are required and propagation loss becomes a critical issue, are the preferred domain of CROWs fabricated with large ring resonators, while at data rates in the terabit range, PhCWs remain the leading technology.

2D Photonic crystal thermo-optic switch based on AlGaAs/GaAs epitaxial structure

The realisation of a thermo-optically controlled symmetric Mach- Zehnder interferometer switch based on an AlGaAs/GaAs epitaxial waveguide structure operating at wavelengths in the region of lambda = 1550 nm is reported. The device is based on a very compact two-dimensional photonic crystal channel waveguide structure. The measured and simulated transmission spectra for the devices are in good agreement. The Pi-phase shift switching power for the device is as low as 42 mW.

A UNIVERSAL DAMAGE INDUCED TECHNIQUE FOR QUANTUM WELL INTERMIXING

We report a novel technique for quantum well intermixing which is simple, reliable and low cost, and appears universally applicable to a wide range of material systems. The technique involves the deposition of a thin layer of sputtered SiO2 and a subsequent high temperature anneal. The deposition process appears to generate point defects at the sample surface, leading to an enhanced intermixing rate and a commensurate reduction in the required anneal temperature. Using appropriate masking it is possible to completely suppress the intermixing process, enabling large differential band gap shifts (over 100 meV) to be obtained across a single wafer.

Monolithic integration via a universal damage enhanced quantum-well intermixing technique

A novel technique for quantum-well intermixing is demonstrated, which has proven a reliable means for obtaining postgrowth shifts in the band edge of a wide range of III-V material systems. The technique relies upon the generation of point defects via plasma induced damage during the deposition of sputtered SiO/sub 2/, and provides a simple and reliable process for the fabrication of both wavelength tuned lasers and monolithically integrated devices. Wavelength tuned broad area oxide stripe lasers are demonstrated in InGaAs-InAlGaAs, InGaAs-InGaAsP, and GaInP-AlGaInP quantum well systems, and it is shown that low absorption losses are obtained after intermixing. Oxide stripe lasers with integrated slab waveguides have also enabled the production of a narrow single lobed far field (3/spl deg/) pattern in both InGaAs-InAlGaAs, and GaInP-AlGaInP devices. Extended cavity ridge waveguide lasers operating at 1.5 /spl mu/m are demonstrated with low loss (/spl alpha/=4.1 cm/sup -1/) waveguides, and it is shown that this loss is limited only by free carrier absorption in waveguide cladding layers. In addition, the operation of intermixed multimode interference couplers is demonstrated, where four GaAs-AlGaAs laser amplifiers are monolithically integrated to produce high output powers of 180 mW in a single fundamental mode. The results illustrate that the technique can routinely be used to fabricate low-loss optical interconnects and offers a very promising route toward photonic integration.

Electro-optic-waveguide frequency translator in LiNbO 3 fabricated by proton exchange

An optical waveguide phase modulator has been fabricated on X-cut LiNbO(3) by using proton exchange in benzoic acid. The phase modulator was operated as a serrodyne optical-frequency translator with shifted-signal to imagesignal discrimination of 52 dB for a 4-MHz frequency shift. The amplitude of the sawtooth driving signal was 10 V peak to peak. Application of a de bias voltage of either polarity was found to cause a substantial reduction in transmitted-light intensity.

Monolithic integration in InGaAs-InGaAsP multiple-quantum-well structures using laser intermixing

The bandgap of InGaAs-InGaAsP multiple-quantum-well (MQW) material can be accurately tuned by photoabsorption-induced disordering (PAID), using a Nd:YAG laser, to allow lasers, modulators, and passive waveguides to be fabricated from a standard MQW structure. The process relies on optical absorption in the active region of the MQW to produce sufficient heat to cause interdiffusion between the wells and barriers. Bandgap shifts larger than 100 meV are obtainable using laser power densities of around 5 W/spl middot/mm/sup -2/ and periods of illumination of a few minutes to tens of minutes. This process provides an effective way of altering the emission wavelengths of lasers fabricated from a single epitaxial wafer. Blue shifts of up to 160 nm in the lasing spectra of both broad-area and ridge waveguide lasers are reported. The bandgap-tuned lasers are assessed in terms of threshold current density, internal quantum efficiency, and internal losses. The ON/OFF ratios of bandgap-tuned electroabsorption modulators were tested over a range of wavelengths, with modulation depths of 20 dB obtained from material which has been bandgap-shifted by 120 nm, while samples shifted by 80 nm gave modulation depths as high as 27 dB. Single-mode waveguide losses are as low as 5 dB/spl middot/cm/sup -1/ at 1550 mm. Selective-area disordering has been used in the fabrication of extended cavity lasers. The retention of good electrical and optical properties in intermixed material demonstrates that PAID is a promising technique for the integration of devices to produce photonic integrated circuits. A quantum-well intermixing technique using a pulsed laser is also demonstrated.

Characterization of proton‐exchange slab optical waveguides in z‐cut LiNbO3

We report the results of a systematic study on planar optical waveguides fabricated in z‐cut LiNbO3 by proton exchange in benzoic acid. It was found that the refractive index varied with depth and could be accurately modeled by a step index profile with Δn=0.126. Diffusion coefficients have been calculated from mode effective refractive index measurements, assuming a step index profile, and hence a value for the activation energy for the proton exchange process has been deduced. The lowest measured optical propagation loss in single‐mode waveguide at a 633‐nm wavelength was 2.4 dB/cm.